Progress is being made for substitutes for a compressor in a room air conditioner, a car air conditioner, and the like, for a bearing of a transmission rotating shaft in an automobile, construction machinery, and the like, and metal rolling bearings in a thrust bearing.
Various proposals as substitutes for the sliding of a metal rolling bearing include not only resin materials, but also sintered materials. However, load-bearing capacity and heat resistance are not sufficient in resin materials alone, and sintered materials have a seizing problem when oil is depleted. As a countermeasure thereto, there has been proposed a multilayered bearing in which copper-based sintered layer is disposed on the surface of a steel plate and a resin material is impregnated in the sintered layer, and also known is a polytetrafluoroethylene (hereinafter notated as PTFE) resin composition blended with various fillers and coated as a sliding face of the multilayered bearing. However, PTFE resin has poor creep resistance and load-bearing capacity is insufficient.
Also known are multilayered bearings coated with polyether ether ketone (hereinafter notated as PEEK) resin, polyamide resin, liquid crystal polymer, and the like, in lieu of PTFE resin. For example, a plain bearing has been proposed in which an intermediate layer of bronze is sintered onto a metal base, a lining material layer is applied to the sintered product, heat and pressure are applied to the lining, the intermediate layer, and the base, and the lining is a substance having a composition comprising a mixture of 60 to 90 wt % PEEK resin, 15 to 3.7 wt % PTFE resin, 5 to 1.3 wt % graphite, and 20 to 5 wt % bronze (see patent document 1). Also proposed is a wet-type multilayered sliding member composed of a surface lay essentially comprising a backing metal layer, a porous sintered layer disposed on the backing metal layer, and a PEEK resin impregnated in and coated on the porous sintered layer (see patent document 2). Additionally, there is also proposed a sliding member for wet-type thrust bearing composed of 10 to 45 wt % carbon fiber, 0.1 to 8.5 wt % PTFE resin, and the remainder essentially being a PEEK resin or polyphenylene sulfide (hereinafter notated as PPS) resin in a porous sintered layer with a backing metal (patent document 3).
Also, the compressor has a rotating member for driving the compression mechanism, and this rotating member is supported by a bearing. The plain bearing for supporting the rotating member for driving the compression mechanism is required to have precise rotation accuracy, to have excellent load-bearing capacity and creep resistance in order to stably obtain low rotational torque, to have no dimensional change under high pressure, and to have other advantageous properties. Examples of a plain bearing used in the same application include the sliding member of patent document 2.
The structure of a so-called cradle-type pump (may hereinafter be referred to merely as “pump”) is well known as a variable-capacity piston pump used in a hydraulic pressure generator for a hydraulic circuit. In a cradle-type pump, a cylinder block for accommodating a piston is integrally rotated together with a rotating shaft, and the cradle makes sliding contact with a cradle guide and is supported so as to allow sloping with respect to the rotating shaft and is in contact with the sloped surface of the cradle via a shoe coupled to the end part of the piston. Therefore, the piston reciprocates with a stroke provided in accordance with the slope of the cradle in accompaniment with the rotation of the rotating shaft, and is made to demonstrate a pumping effect. The discharge capacity of the pump determined by the stroke difference can be constantly modified by controlling the slope angle of the cradle in relation to the rotating shaft by using hydraulic pressure or the like.
However, when a cradle composed of, e.g., an aluminum material (including aluminum alloys) is brought into sliding contact and held in a cradle guide composed of the same aluminum material, the two components create sliding-contact friction when used for constantly controlling the slope angle of the cradle in relation to the rotating shaft using hydraulic pressure or the like, resulting in seizing or other problems. For this reason, a means has been adopted in which a thrust bushing made of synthetic resin is interposed between the cradle and cradle guide.
Well-known examples of a thrust bushing serving as a cradle guide include metal thrust bushings in which a resin film has been applied to the sliding surface, and a thrust bushing composed of nylon (polyamide resin), polyacetal resin, PTFE resin, and other sliding resins (see patent document 4).
Also known are variable-capacity piston pumps in which the cradle and/or cradle guide composed of an aluminum material is coated with an ethylene tetrafluoroethylene copolymer (ETFE) resin, a tetrafluoroethylene-hexafluoropropylene copolymer (FEP) resin, a PTFE resin, or another fluororesin (see patent document 5).
A thrust bushing in which a sintered copper film is formed on the surface of an iron base material, and a thrust busing in which a resin film is furthermore formed on the surface of the sintered film are also known as thrust bushings that serve as a cradle guide (see patent document 6).
A sliding screw device for converting rotational motion into rectilinear motion has an advantage in what it can be compactly designed in comparison with a ball screw device, and is often used in feed devices for industrial machinery, positioning devices, and the like. In a sliding screw device that uses a nut made of a copper alloy or another metal, an increase in torque and seizing are concerns due to depletion of applied oil or grease, and periodic maintenance is therefore required. Also, such cannot be used in a vacuum, in water, or in other environments where oil or grease cannot be applied. In view of the above, sliding screw devices and the like that use a resin nut are being developed for making usage possible without lubrication, making the device maintenance free, and achieving other purposes.
There are examples of a nut made entirely of resin or a nut in which the groove portion of the threading as the sliding portion is made of resin. One such example is a resin nut in which the groove portion of the threading (or the entire nut) to be threaded onto a screw shaft is formed from a PPS resin composition in which PTFE resin and an organic resin powder that does not melt at 280° C. are blended with PPS resin (see patent document 7). Another proposal is a sliding screw device provided with a nut that moves in a relative manner while axially sliding on the screw shaft in accompaniment with the rotation of the screw shaft, the nut having a powder-coated film of an aromatic polyimide resin formed on at least the female threading part thereof (see patent document 8).
A proposed example of a nut composed of a metal portion and a resin portion is a flanged nut that is threaded onto a screw shaft and that moves in a relative manner in the axial direction with the screw shaft, the external peripheral portion including the flange being formed from metal, the internal peripheral portion that is threaded onto the screw shaft being formed from a lubricating resin, and means to detent and retain the nut being disposed between the external peripheral portion and the internal peripheral portion (see patent document 9).
Additionally, there has been proposed a method for manufacturing a resin nut that uses a mold for injection molding provided with: a fixed mold having a molding surface for molding one end face of the resin nut or one end face and the vicinity thereof; a movable mold having a cavity for molding the remaining outside shape surface of the resin nut and capable of moving in the axial direction with respect to the fixed mold; and a core pin provided to the movable mold and in which helical grooves for molding threading grooves are formed in the outside diameter surface. A molten resin is filled into the mold and a resin nut is molded. The resin nut is thereafter removed by opening the metal mold and rotating the core pin (see patent document 10).